Nowadays, as an accurate distance measurement method, a distance measurement method using ultrasonic waves is being used. In this method, ultrasonic waves are emitted from an ultrasonic generator and are caused to strike a measurement object. Ultrasonic waves reflected from the measurement object are detected by an ultrasonic microphone device. This method is a method for calculating the distance to the measurement object from the time taken from the emission to the detection.
For example, Patent Document 1 discloses an ultrasonic generator in which piezoelectric vibrators are mounted on a housing. The ultrasonic generator in Patent Document 1 is formed as an ultrasonic sensor device in which a single device serves as both an ultrasonic generator and an ultrasonic microphone device.
FIG. 9 shows an ultrasonic generator 500 disclosed in Patent Document 1. The ultrasonic generator 500 includes a case 101. A first piezoelectric vibrator 102 and a second piezoelectric vibrator 103 that vibrates in an opposite phase to that of the first piezoelectric vibrator 102 are disposed in the case 101. A space in the case 101 is filled with a flexible filler 105.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-297219
In the above-described distance measurement method, in order to make measurement results more accurate and to lengthen the measurable distance, it is necessary to increase output sound pressure of the ultrasonic generator.
However, in the existing ultrasonic generator 500, there is a limit to how high the output sound pressure can be made. In order to increase the output sound pressure, it is necessary to increase polarization of a piezoelectric vibrator and to increase electric power that is supplied to the piezoelectric vibrator. However, there is a limit to the extent of polarization of the piezoelectric vibrator. If the electric power supply is too large, the piezoelectric vibrator may exceed the fracture limitation. Consequently, there is a limit to how high the output sound pressure can be made.